This invention relates to vibratory conveyors, and, more particularly, to a drive and drive controller system for powering such a vibratory conveyor.
Vibratory conveyors are used industrially to move fragile, small, or light weight articles from one point to another. For example, in the packaging of fragile food products such as cookies or potato chips, the food product is received from a central location such as a cooking oven and is conveyed to a plurality of work stations, whereat a packaging machine fills and seals a package with the food product. The vibratory conveyor system is necessary because the food products are fragile and may not be readily grasped at this stage of production. An example of a vibratory conveyor used in this fashion is provided in U.S. Pat. No. 3,731,787.
One type of vibratory conveyor includes a frame, a tray for receiving a product, the tray being mounted to the frame by a resilient means usually comprising a flexure inclined to the vertical, and a drive system to vibrate the tray within the path defined by the flexure. An article resting on the tray is thrown forward in the direction of conveyance with each cycle of the drive system. The tray is typically vibrated at a rate of 600-1400 cycles per minute, and an article resting on the tray experiences an equal rate of throws or vibrational impulses.
A variety of approaches have been used to provide the vibratory motion in such conveyors. In one approach, the rotary motion of a conventional rotary motor is converted to linear motion by an eccentric mechanical linkage. Such an approach has significant disadvantages, particularly as to noise created by the drive system, wear of the linkage, and the need to lubricate the moving parts. The necessity of introducing lubricants into a clean environment such as used to convey food products is highly undesirable. In one alternative approach, termed a direct-attraction drive, the use of an eccentric mechanical linkage is avoided by providing a drive having two pole pieces, one attached to the frame of the conveyor and the other attached to the tray. In a direct-attraction drive, the flat faces of the pole pieces are perpendicular to the direction of action of the drive. One of the pole pieces may be permanently magnetized or magnetized by a direct current, and the other is an electromagnet alternately magnetized and demagnetized by an alternating current in a coil wound on the pole piece, so that the two pole pieces are alternately drawn together and allowed to relax apart under the restoring force of the flexure. With this arrangement, the amplitude of vibration is relatively fixed by the initial spacing between the pole pieces. The length of travel of the pole pieces is not inherently self-limiting, so that the two pole pieces may clap together, causing significant damage to the drive.
To overcome the problems associated with such drives, some principles of linear drives have been adapted to vibratory conveyors. In a linear drive, two pole pieces whose flat faces are parallel to the direction of action of the drive move parallel to each other while maintaining a relatively constant lateral spacing. The motive force is provided by alternately magnetizing and demagnetizing one of the pole pieces to attract the pole pieces toward each other along the direction of action, thereby producing motion parallel to the direction of action of the drive. The amplitude of the vibration may be significantly increased over that of conventional direct-attraction drives, and various modifications have been proposed to the design of the pole pieces to further increase the available vibrational amplitude.
While the various means to provide vibration to vibratory conveyors have certain advantages, all prior drives suffer from the common disadvantage of a lack of complete controllability. Use of a rotary motor allows the frequency of vibration to be readily controlled by adjusting the motor speed, but the amplitude of the vibration is not readily controlled, except by changing the linkage and eccentric mechanically. In the conventional direct-attraction drive, and the conventional linear drive, operating from available 50 or 60 cycle alternating current, the vibrational frequency may not be readily controlled. There have been a few attempts to utilize complex electronics to vary the frequency of the vibration, but for the most part the vibrational frequency has been limited to that produced by a motor operating at 50 or 60 cycles per second, or an integral fraction thereof, and vibrating a supported mass (the tray). It is highly desirable to have the ability to drive the tray at or near its natural frequency, as the power requirements of the conveyor are thereby reduced. The lack of complete controllability of prior vibrating conveyors thus does not permit an optimal conveying of the articles in the tray, and causes inefficiencies.
For some applications, the vibratory conveyor works satisfactorily even without complete control of vibrational amplitude, frequency and power input. In other applications such complete control would be desirable to tune the conveyor to minimize the required input power, while optimizing the control of movement of the product. Accordingly, there exists a need for a drive and drive controller system for use with vibratory conveyors, to provide an energy efficient power source wherein amplitude, frequency, and power input may be readily and arbitrarily controlled using reliable apparatus elements. The present invention fulfills this need and further provides related advantages.